Field of the Invention
The present invention relates to an image processing technique to eliminate unevenness in density of an image.
Description of the Related Art
As an image printing system that is used in a color image forming apparatus, such as a color printer and a color copy machine, an electrophotographic system is known. In the electrophotographic system, a latent image is formed by exposing a photoconductor drum to laser beams or by a light-emitting element, such as an LED, and the latent image is developed by toner, which is a charged color material. An image is printed by transferring the image developed by toner to a printing medium, such as paper, and fixing the image. As an electrophotographic image forming apparatus, there is a tandem color image forming apparatus that includes development counters and photoconductor drums in the same number as that of toner colors and which sequentially transfers images of different color planes onto an image conveyance belt or onto a printing medium. In the tandem color image forming apparatus, it is known that a deviation of position (called “registration deviation”) in an image for each color component occurs. As a method of suppressing the registration deviation, there is a technique to correct image data by measuring the magnitudes of inclination and bend of a laser beam scan line by using an optical sensor and cancelling out them (e.g., see Japanese Patent Laid-Open No. 2004-170755). In this technique, processing to shift the pixel within image data in the sub scanning direction by one pixel each time (phase change processing) is performed.
In the recent image forming apparatus, it is common to perform printing by developing print data (PDL data) from a host computer into bitmap data. In the case where development into bitmap data is performed with a resolution higher than the resolution of the image forming apparatus, a method is known which reproduces an image with a high resolution in a pseudo manner by using a spot multiplexing technique (e.g., see Japanese Patent Laid-Open No. H04-336859(1992)). The spot multiplexing technique is a technique to form a dot with a resolution higher than the resolution of the image forming apparatus by forming dots that overlap each other at an intermediate potential level for adjacent pixels to turn the potential level at the overlapping portion to a high level.
Here, in the case where the method of Japanese Patent Laid-Open NO. 2004-170755 which corrects image data so as to cancel out distortion resulting from the bend of a scan line and the method of Japanese Patent Laid-Open No. H04-336859(1992) which reproduces an image with a high resolution in a pseudo manner by making use of spot multiplexing are made use of at the same time, there is a possibility that unevenness occurs in an image on a printing medium. The reason is as follows. To begin with, the above-described phase change processing causes a difference in the pixel position. In order to reduce the difference in the pixel position, it is sufficient to perform the phase change processing for image data with a higher resolution. Consequently, it is considered to perform, after performing the phase change processing for image data with a high resolution, processing to convert the image data after the processing into image data with a low resolution for the above-described spot multiplexing. However, in this case, in the image data after the conversion into a low resolution, the kind and shape of halftone dots that appear change with the position in the main scanning direction at which the difference in the pixel position occurs due to the phase change processing as a boundary. As a result of this, unevenness in color occurs in the image on the printing medium. For this problem, a technique has been proposed which suppresses a change in density from occurring before and after the phase change processing by limiting two vector components representing the period of halftone dot to a combination of an even number and an odd number in the dither matrix that is used in halftone processing (see Japanese Patent Laid-Open No. 2013-236180).
As described above, in the case where image data is converted into image data with a low resolution by pseudo high-resolution processing after performing the halftone processing and the phase change processing with a high resolution, with a conventional common dither matrix, unevenness occurs in an image on a printing medium. The above-described technique of Japanese Patent Laid-Open No. 2013-236180 resolves this problem under a certain condition, but the screen ruling and the angle of the dither matrix that is used are restricted and there is a case where moire between colors occurs.